Delayed sleep phase syndrome (DSPS) is a condition characterized by a delay in the timing of sleep onset and waking. Generally present from an early age, DSPS usually produces considerable difficulties for those who are required to function on a 9-to-5 schedule. Most subjects have tried unsuccessfully to correct their sleep-wake cycle many times. One type of currently available treatment, chronotherapy, involves delaying the timing of sleep onset further each day until it reaches the desired clock time. Although this is quite effective in some cases, it is inconvenient and the benefits are often transient. The object of the present study is to recruit subjects who complain of DSPS, to determine the clinical and demographic characteristics of these individuals, and to try to correct their circadian rhythm abnormalities by judicious use of exposure to bright light and darkness, and by using the short-acting benzodiazepine, triazolam. Both of these strategies are derived from studies of circadian rhythms in animals, the timing of which is ordinary influenced by the light-dark cycle. In recent years we have found that light of intensity greater than ordinary indoor lighting has a variety of effects on brain function. For example, it is capable of suppressing melatonin, reversing depression in seasonal affective disorder, altering norepinephrine and serotonin metabolism, and affecting resting metabolic rate and immune function. There is also evidence that bright light can influence the timing of circadian rhythms in humans. We identified 93 subjects with DSPS and noted their reported patterns of sleeping and waking, as well as the difficulties encountered as a result of their sleep problem. We have studied sleep architecture in 11 cases and have not found patients to be suffering from another type of sleep disturbance. Three patients have been treated with both active (bright light in the morning plus dark goggles in the evening) and control (dimmer light in the morning and transparent goggles in the evening) light treatment conditions, and it appears that the active but not the control condition has a beneficial effect on the timing of sleeping and waking. However, the results are preliminary and more subjects will have to run before a definitive statement can be made as to the value of bright light in the treatment of DSPS. We have yet to explore the value of triazolam in this condition.